Add support for $locf aggregation operator.

See #4139
Original pull request: #4182.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/EvaluationOperators.java

@@ -16,7 +16,6 @@
 package org.springframework.data.mongodb.core.aggregation;
 
 import org.bson.Document;
-
 import org.springframework.data.mongodb.core.query.CriteriaDefinition;
 import org.springframework.util.Assert;
 
@@ -24,6 +23,7 @@ import org.springframework.util.Assert;
  * Gateway to {@literal evaluation operators} such as {@literal $expr}.
  *
  * @author Divya Srivastava
+ * @author Christoph Strobl
  * @since 3.3
  */
 public class EvaluationOperators {
@@ -86,6 +86,15 @@ public class EvaluationOperators {
 			return usesFieldRef() ? Expr.valueOf(fieldReference) : Expr.valueOf(expression);
 		}
 
+		/**
+		 * Creates new {@link AggregationExpression} that is a valid aggregation expression.
+		 *
+		 * @return new instance of {@link Expr}.
+		 */
+		public LastObservationCarriedForward locf() {
+			return usesFieldRef() ? LastObservationCarriedForward.locfValueOf(fieldReference) : LastObservationCarriedForward.locfValueOf(expression);
+		}
+
 		/**
 		 * Allows the use of aggregation expressions within the query language.
 		 */
@@ -152,4 +161,45 @@ public class EvaluationOperators {
 		}
 	}
 
+	/**
+	 * Sets {@literal null} and missing values to the last non-null value.
+	 *
+	 * @since 4.0
+	 */
+	public static class LastObservationCarriedForward extends AbstractAggregationExpression {
+
+		private LastObservationCarriedForward(Object value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$locf";
+		}
+
+		/**
+		 * Creates new {@link EvaluationOperatorFactory.Expr}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link EvaluationOperatorFactory.Expr}.
+		 */
+		public static LastObservationCarriedForward locfValueOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null");
+			return new LastObservationCarriedForward(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link LastObservationCarriedForward}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link LastObservationCarriedForward}.
+		 */
+		public static LastObservationCarriedForward locfValueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null");
+			return new LastObservationCarriedForward(expression);
+		}
+	}
+
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/spel/MethodReferenceNode.java

@@ -251,6 +251,9 @@ public class MethodReferenceNode extends ExpressionNode {
 		map.put("toString", singleArgRef().forOperator("$toString"));
 		map.put("degreesToRadians", singleArgRef().forOperator("$degreesToRadians"));
 
+		// expression operators
+		map.put("locf", singleArgRef().forOperator("$locf"));
+
 		FUNCTIONS = Collections.unmodifiableMap(map);
 	}
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/EvaluationOperatorsUnitTests.java

@@ -23,6 +23,7 @@ import org.junit.jupiter.api.Test;
  * Unit tests for {@link EvaluationOperators}.
  *
  * @author Mark Paluch
+ * @author Christoph Strobl
  */
 class EvaluationOperatorsUnitTests {
 
@@ -32,4 +33,11 @@ class EvaluationOperatorsUnitTests {
 		assertThat(EvaluationOperators.valueOf("foo").expr().toDocument(Aggregation.DEFAULT_CONTEXT))
 				.isEqualTo("{ $expr: \"$foo\" }");
 	}
+
+	@Test // GH-4139
+	void shouldRenderLocfCorrectly() {
+
+		assertThat(EvaluationOperators.valueOf("foo").locf().toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo("{ $locf: \"$foo\" }");
+	}
 }

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/SpelExpressionTransformerUnitTests.java

@@ -1250,6 +1250,11 @@ public class SpelExpressionTransformerUnitTests {
 	void shouldTsSecond() {
 		assertThat(transform("tsSecond(saleTimestamp)")).isEqualTo("{ $tsSecond: \"$saleTimestamp\" }");
 	}
+
+	@Test // GH-4139
+	void shouldRenderLocf() {
+		assertThat(transform("locf(price)")).isEqualTo("{ $locf: \"$price\" }");
+	}
 	
 	private Document transform(String expression, Object... params) {
 		return (Document) transformer.transform(expression, Aggregation.DEFAULT_CONTEXT, params);